Molten salts (MS), usually a mixture of nitrates, are known to be used for thermal energy storage in concentrated solar power plants (CSP plants).
In CSP plants, solar energy is captured in a concentrated way by means of mirrors, and made to heat a fluid that serves to produce steam which, in turn, is used to produce electric power by means of a turbine and generator system.
It will be understood that sun radiation is not available continuously. Therefore, the thermal energy recovered from solar radiation during day time, is stored during day and used during night time so as to allow the resulting power to be available at all times.
The most common method to store the captured sun radiation energy is to heat a mass of molten salts, mostly a mixture of nitrates, during the day, while using these hot molten salts, either directly or indirectly, for producing steam and there from electric power. Alternatively, the molten salts used for storing the thermal energy are not themselves heated by sun radiation, but via a different heat transfer fluid that itself is subjected to the solar heating.
The heat storage systems used to store molten salts usually comprises one or more paired tanks (named “hot” and “cold” storage tanks). During molten salts heating, the molten salts are transferred from the cold tank to the hot one. When the heat is recovered, molten salts flow from the hot tank to the cold tank.
As an alternative to the two tank storage system, thermocline storage systems can be used. A thermocline storage tank system is a single-tank system containing both the hot and cold molten salts. This type of system relies on thermal buoyancy to maintain thermal stratification and discrete hot and cold thermal regions inside the tank. Since the density of high temperature molten salts is lower than that of low temperature molten salts, the first volume of high temperature molten salts stratifies on the top of the low temperature molten salts, thus forming a natural interface region extending substantially horizontally. It will be understood that, depending on the relative volumes of the high and low temperature molten salts, this interface moves substantially vertically relative to the storage tank. This system represents an economical alternative to the two-tank storage system.
According to the different CSP plants schemes, the “cold” tank (or the low temperature volume in a thermocline tank) operates within a temperature range varying from 270° C. to 400° C., while the hot tank (or volume) temperature may reach a maximum value of 550° C.
CSP plants are today sized to produce electricity with an electric power output which ranges from 10 MW to 500 MW. Since the efficiency of the power generation system ranges from 30% to 50% and it is requested to produce electricity for 6 or 12 hours when the sun is not available, CPS plants need to store an amount of thermal energy in the range of 100 MWh to 20,000 MWh.
Accordingly a typical storage tank diameter is in the range of 15 m to 50 m, with the tank height being in the range of from 7 m to 18 m. Thus, the overall surface to be insulated typically ranges from 700 m2 to 7000 m2.
It will be understood that it is imperative to minimize heat loss as much as possible, preferably to less than few degrees ° C. per day of the total heat stored. To this end, generally up to 900 mm thickness of external insulation are required (from 350 m3 to 3500 m3 of insulation volume being required). This is a serious burden on the investment in CSP plants. Further, MS tanks employed in CSP plants have several disadvantages and difficulties due to the combined effect of thermal expansion and contraction of the metallic walls. Also, at the bottom of the tank an insulation material is required between the tank and the (normally concrete) foundation, so as to prevent said concrete foundation from reaching too high a temperature.
A background reference is U.S. Pat. No. 4,523,629. Therein a thermocline storage tank for molten salts is provided with an insulated barrier member at the interface region between the hot and cold liquids. Provisions are made (e.g. by correctly selecting the density of the barrier member) so as to make the barrier member float on the lower placed liquid, i.e. so as to allow it to move with the interface between the hot and cold fluids in the tank. Additionally, the document refers to the use of an internal insulation, so as to allow lower cost insulation on the outside. The internal insulation is submerged in the molten salt, and is wetted by it.
Another background disclosure on a molten salts storage tank, is WO 2011/116040. Herein an inner liner is provided that repeatedly expands and contracts during the thermal cycling of the storage system.
A still remaining challenge in the field is to improve the design of storage tanks for molten salts in respect of thermal insulation. This with a view to enabling better operating performance of the materials involved, and in view of enabling lower investment costs.